Coated molybdenum mesh screen for ferrous metal casting molds

ABSTRACT

A CASTING METHOD AND APPARATUS WHEREIN CAST IRON IS POURED THROUGH A COATED, EXPANDED METAL, MOLYBDENUM SCREEN TO REMOVE FOREIGN MATERIAL FROM THE POURED CAST IRON. THE MOLYBDENUM SCREEN, THAT IS HIGHLY SOLUBLE IN CAST IRON AT THE POURING TEMPERATURES, IS PROTECTED BY A CERAMIC-LIKE COATING SO THAT IT WILL NOT BE IN DIRECT CONTACT WITH THE CAST IRON.

Aug. 10, 1971 3, w FOSTER ETAL 3,598,3

COATED MOLYBDENUM MESH SCREEN FOR FERROUS METAL CAST G M0 Filed 061}. 19

&\\\\\\ 1N VENTORS 3 598 732 COATED MoLYBnENuir MESH SCREEN FOR FERROUS METAL CASTING MOLDS Daniel W. Foster, Walled Lake, and William G. Scholz,

Ann Arbor, Mich., assignors to American Metal Climax, Inc.

Filed Oct. 14, 1969, Ser. No. 866,353 Int. Cl. B01d 39/10 US. Cl. 210-499 6 Claims ABSTRACT OF THE DISCLOSURE A casting method and apparatus wherein cast iron is poured through a coated, expanded metal, molybdenum screen to remove foreign material from the poured cast iron. The molybdenum screen, that is highly soluble in cast iron at the pouring temperatures, is protected by a ceramic-like coating so that it will not be in direct contact with the cast iron.

CROSS-REFERENCE TO RELATED APPLICATION This application is related in subject matter to the copending application entitled Screen for Molten Metal, Ser. No. 631,260, filed Apr. 17, 1967 in the name of Gordon D. McArdle now Pat. No. 3,508,601.

BACKGROUND OF THE INVENTION This invention relates to an improved coated screen for use in the pouring of cast iron and to an improved method of making such a screen.

In the aforenoted patent application of Gordon D. McArdle the advantages of pouring non-ferrous metals through a molybdenum screen to remove drosses and slag from the poured metal are stressed. These advantages also may be realized if ferrous metals are poured through such a screen. Molybdenum was previously thought to be unsatisfactory for such an application, however, due to its high solubility in cast iron at normal pouring temperatures. That is, a molybdenum screen through which molten cast iron was poured would rapidly dissolve in the cast iron. Although perforated ceramic plates have been proposed for screening poured metals, such screens are highly brittle, are expensive to make and have a low ratio of opening size to cross-sectional area; hence, they reduce the pouring rates causing other defects.

It is, therefore, a principal object of this invention to provide a screen for use in pouring ferrous metals.

It is another object of the invention to provide an improved coated molybdenum screen for pouring ferrous metals.

It is a further object of the invention to provide a method for coating a molybdenum screen so that it may be used in the pouring of cast iron or other ferrous metals.

SUMMARY OF THE INVENTION A screen embodying this invention is particularly adapted for removing impurities from molten cast iron as it is poured. The screen comprises a molybdenum screen having a plurality of strands defining openings. These strands are readily soluble in the poured cast iron at its pouring temperatures. A coating is formed on the molybdenum screen from a material having a substantially higher melting point than the temperature of the poured cast iron. The coating on the screen is sufficient to prevent direct contact between the poured cast iron and the molybdenum of the screen.

A method embodying this invention is particularly adapted for coating a screen as set forth in the preceding paragraph. In this method a mixture of the coating ma- 3,598,732 Patented Aug. 10, 1971 terial and a solvent that is adapted to each the molybdenum is placed upon the molybdenum screen and is dried.

BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring in detail to FIGS. 1 and 2, the reference numeral 11 indicates a sand mold for use in the casting of articles formed from ferrous metals, such as any of the cast irons. The mold 11 is comprised of a drag 12 formed from casting sand ram packed into a drag flash 13. Supported above the drag 12 and drag flask 13 is a cope 14, which is also formed from casting sand ram packed into a cope flask 15. The cope 14 and drag 12 engage each other along a parting line indicated generally by the reference numeral 16.

A cavity 17 is formed in the drag 12 and cope 14 in a known manner which cavity has the shape of the article to be cast. As is well known in casting techniques, one or more cores may be inserted into the cavity 17 if openings are desired in the finished casting. A runner 18 is formed in the drag 12 adjacent the parting line 16 and opening at one end into the cavity 17. The other end of the runner 18 terminates in an enlarged cavity 19 which is adjacent to the lower end of a sprue 21 formed in the cope 14 and may be considered to be an extension thereof. The sprue 21 extends vertically through the cope 14 and terminates at its upper end in an enlarged pouring basin 22. At the side of the cavity 17 opposite to the runner 18, a riser 23 is formed which extends through a portion of the drag 12 and through the cope 14. The riser 23 terminates in the upper surface of the cope 14.

During casting the molten cast iron is poured into the pouring basin 22 from a ladle or the like at a temperature that will be considerably lower than the melting point of molybdenum. This molten cast iron passes down the sprue 21, across the runner 18 and into the cavity 17. Sufiicient metal is poured to fill the riser 23 and insure that the complete volume of the casting 17 is filled. The cast iron thus poured contains slag or other drosses which might enter the cavity 17 and form impurities within the finished cast article. In order to exclude these impurities from the finished casting, a coated expanded molybdenum screen, indicated generally by the reference numeral 25, is positioned within the mold 11 at the parting line 16 and extending across the sprue 21.

Referring now in more detail to FIGS. 3 and 4, the coated screen 25 is formed about a base expanded molybdenum screen. The molybdenum screen is formed with a plurality of generally like size uniformly distributed polygonal openings 26 that are defined by unitary strands 27 of molybdenum. The strands 27 defining adjacent openings 26 are integrally connected to each other at the apexes of the polygonal openings, as clearly shown in FIG. 3. This form of screen may be made by any conventional metal expanding technique.

As is well known, molybdenum is highly soluble in the ferrous metals at the melting or pouring temperatures of these metals. Hence, if the moltencast iron directly contacts the molybdenum screen, it would soon be dissolved and would not serve its intended purpose. For this reason, a coating, indicated generally by the reference numeral 3 28 (FIG. 4), is deposited upon the molybdenum strands 27 and completely encloses these strands while retaining the openings 26. Of course, the size of the openings 26 will be diminished somewhat by the application of the coating 28. In a preferred embodiment of the invention, the molybdenum screen has an approximate mesh thickness of 0.020 inch and a mesh strand width of 0.010 inch. The approximate size of the uncoated openings is 0.280 inch in their long direction and 0.125 inch in their short direction. There are approximately 60 openings per square inch and the open area of the mesh comprises about 85% of its total cross-sectional area.

The coating 28 is of a material that has a higher melting point than the melting point of the poured cast iron. In a preferred embodiment of the invention the coating comprises 80% alundum and 20% silica by weight. The coating is applied by forming a mixture containing approximately 58% by weight of the alundum-silica mixture in the aforenoted percentages and 42% by weight of a sodium hydroxide. The sodium hydroxide was in the strength of 200 grams of sodium hydroxide per liter of water. The coated mesh was allowed to air dry for half an hour and then was transferred to a furnace set at 800 F. or the coated mesh may be directly placed in the 800 F. furnace. The coated mesh was left in the furnace at 800 F. for two hours. In the mixture applied to the screen, small amounts of alundum and silica were dissolved. Upon the subsequent drying and baking in the 800 F. furnace, the dissolved alundum and silica were precipitated and acted as a binder for the remainder of the alunduma'fnd silica so that the coated mesh could be handled without dislodging the coating.

Mesh coated in the aforenoted manner is effective in protecting the molybdenum from dissolving in the poured cast iron. In addition, the coated mesh is eifective in removing slags and drosses from the cast iron as it is poured. If a thicker coating is desired, the aforenoted coating procedure may be repeated as many times as necessary to provide the desired coating thickness.

Under some circumstances, it may be desirable to form a coating that is more rigid and which will lend itself to more handling without disturbing the coating. Such a coating has been formed by sintering or fusing the mesh which has been coated in the aforenoted manner.' To accomplish this, the coated mesh which was dried in the 800 F. furnace was transferred to a hydrogen furnace while still warm so that the sodium hydroxide did not pick up any moisture from the air and was heated for one minute in the hydrogen furnace at 2900 F. This temperature was sufficient to cause sintering or fusion of the coating. The silica which was added to the alundum reduced the fusion temperature of the mixture so that fusion would occur at 2900 F. temperature. The sodium hydroxide attacks the molybdenum mesh slightly and when subjected to the high temperatures cause an etching action which aids in bonding the alundum-silica mixture to the mesh. After fusion the coated mesh is cooled still in the presence of the hydrogen atmosphere until suificiently cool to bring into the presence of the atmosphere. The resulting mesh was completely covered and the coating was very hard and adherent. As has been previously noted, a thicker coating can be applied by repeating the aforenoted procedure any number of times. The coating thickness on the single coating procedure is about 0.002 inch and a coating thickness of about 0.006 inch results if the procedure is repeated twice.

What is claimed is:

1. A screen for removing impurities from molten cast iron as it is poured comprising a molybdenum screen having a plurality of strands defining openings, said strands being readily soluble in the poured cast iron at its pouring temperature, and a coating upon the molybdenum screen, said coating being formed from a material having a substantially higher melting point than the temperature of the poured cast iron and coating said screen sufliciently to prevent direct contact between the poured cast iron and the molybdenum screen.

2. A screen as set forth in claim 1 wherein the coating comprises a non-metallic material.

3. A screen as set forth in claim 2 wherein the coating is a ceramic material.

4. A screen as set forth in claim 3 wherein the ceramic material comprises a mixture of alundum and silica.

5. A screen as set forth in claim 4 wherein the alundum comprises approximately by weight of the mixture and the silica comprises approximately 20% by weight of the mixture.

6. A screen as set forth in claim 3 wherein the screen has a thickness of about 0.020 inch and the uncoated screen has approximately of its area open, the coating being approximately 0.002 to 0.006 inch thick.

References Cited UNITED STATES PATENTS 5/1958 Hoefer 164-358 4/1970 McArdle 164-358 U.S. Cl. X.R. 

